Add type checking to accessors for proto fields.
Add type checking to accessors for proto fields.

We know apriori what the type of fields
should be in a proto message.
In this cl we declare the input and return types
of the message fields at compile time.
This should help with compile time type checking.

PiperOrigin-RevId: 332272149

qitab/cl-protobufs,qitab/cl-protobufs